Traumatic brain injury (TBI) remains a major cause of death and long-term morbidity. Cerebral edema is a common and ominous sequel of severe TBI which results from loss of blood-brain-barrier (BBB) integrity and hemorrhage in the affected zone. This application is predicated on recent data showing that tPA-/- mice are relatively protected from developing cerebral edema and cortical necrosis after TBI and our finding that tPA increases BBB permeability both directly and indirectly by inducing vasorelaxation through extra-fibrinolytic mechanisms that involve signal transduction through the low density lipoprotein related receptor (LRP) and the integrin avb3. Here, we propose to study how the extra-fibrinolytic activities of tPA modulate BBB permeability, vasorelexation, cerebral edema and neurological outcome in experimental TBI. Our approach includes basic research into the mechanism of tPA-mediated signal transduction using antagonists and tPA variants that dissociate its vasoactive and catalytic properties through three inter-related specific aims. In Specific Aim 1 we will study the molecular determinants required to form complexes between avb3 and LRP and mechanism by which tPA disrupts these complexes and induces BBB permeability and vasorelaxation. In Specific Aim 2 the effect of intravascular thrombosis on the development of post-traumatic brain injury will be elucidated using approaches to isolate the fibrinolytic and signal transduction activities of tPA. In Specific Aim 3 the effect of antagonists to LRP and avb3 and tPA variants selectively lacking fibrinolytic or extra-fibrinolytic function in the sequelae of TBI will be examined in wild type and tPA-/- mice. Also, a new approach to deliver tPA to traumatized vessels will be evaluated using platelet-tPA expressing transgenic mice. Together, these studies will provide new understanding of the role of tPA in mediating CNS injury and novel cellular targets and new formulations of tPA that may improve clinical outcome.